The police officers were hit when explosives in the second vehicle detonated as they were responding to the first blast, US military sources said.
Police and hospital officials said four people were killed in the explosion and nine others injured.
"This place has witnessed several bombings before and we fear that violence will come back after a period of quiet," Mohammed Nasir, a resident, said.
In a second attack, a roadside bomb hit an Iraqi army lorry loaded with ammunition in the Yarmouk neighbourhood struck, killing three soldiers and injuring four civilians.
Two other explosions targeting the security forces occurred in eastern Baghdad and in the central Karrada district where two civilians were killed and seven other people were wounded.
Security take over
Violence has dropped in Iraq over the past 18 months as bomb attacks have decreased across the country.
However, local police forces are still being targeted as they take over more control of the security situation from the US.
US troops are due to withdraw from Iraq by the end of 2011 as stated by the security pact between Washington and Baghdad.
Also on Monday, the US military said that a soldier died the day before of a non-combat related injury near Samarra north of Baghdad.